One of the features of the present invention is the modulation of a beam of electrons which is brought to a focus and directed at a thin thermoplastic film disposed on a conductive substrate. This expedient leads to the deposition of a track of charge on the thermoplastic film and the excess pressure which is exerted on the surface of the film by virtue of the forces between the deposited charge and the conductive substrate produces a local radius of curvature, which is determined by the local charge density, after the film is softened by heating. Accordingly the contours of the film can be formed in accordance with the time variation of the intensity of the beam and subsequent to the deposition of charge and the heating of the film, which may occur either before or after the deposition of charge, the film may be cooled so as to freeze the film into the contoured shape in which it is formed by the combined effects of the deposition of charge and the softening of the film by heating.
This method of recording was, so far as is known, originally developed by William E. Glenn, and is disclosed for example in U.S. Pat. No. 3,113,179 issued 3rd Dec. 1963. In that Patent the apparatus disclosed is arranged to scan the surface of a thermoplastic film by means of an intensity modulated beam of electrons and the scanning is such to produce a pattern of charge which defines a composite diffraction grating which, upon transmission of light therethrough, produces a visible pattern representative of the electric input signals. The aforementioned Patent is confined to a disclosure of the production of a diffraction grating, which is substantially different in nature to the formation of a groove which is one of the objects of the present invention. The particular cross-section of the grooves forming the diffraction pattern is of no particular consequence since it is primarily the spacing and depth of the deformations which are of importance.
Essentially the same method of recording is disclosed in the subsequent U.S. Pat. No. 3,120,991 to Newberry et al issued 11th Feb. 1964.
One proposal for a spatial coherent reproducing system which includes the recording of a signal in a spiral groove is disclosed by Clunis in U.S. Pat. No. 3,287,563. In that Patent there is disclosed a system which includes a thermoplastic disc disposed on an inert translucent base. A video signal is applied to a frequency modulator which controls an electron beam that is directed to strike the surface of a thermoplastic disc. The disc is rotated and translated so as to produce spiral scanning of the film by the beam of electrons. The Patent does not however disclose or suggest any acceptable means for producing a disc which is mechanically playable. No recording parameters are suggested and the method of readout is apparently an optical method for which the translucent substrate is desirable. There is in particular no reference to how the beam should be controlled in practice in order to produce a suitable pattern of charge. This is important as far as another aspect of this invention is concerned, namely that the effective diameter of the electron beam in the direction along the direction of progression of the track of charge is limited by the shortest wavelength that has to be recorded but in practice is very much less than the width of track which is necessary for the formation of a groove that can properly be engaged by a stylus.
Another proposal for forming depressions in a record medium is disclosed in British Pat. Specification No. 1108565 published 3rd Apr. 1968. In that method a laser beam is generated and directed onto the surface of a record medium so as to soften a portion of the surface which is maintained strongly electrically charged. The laser beam is laterally vibrated and the electrical charge is also modulated to produce both depth and lateral modulation of the spiral depression which is formed. This method of recording is fundamentally different from the two methods that have been described hereinbefore and is substantially different from the method employed in the present invention wherein the beam of electrons is used to deposit a charge and the film is heated by other means. Moreover the disclosure in British Pat. No. 1,107,565 gives no guidance on the particular modulation of the laser beam or, more importantly, how to control an electron beam to lay down a track of charge which is wider than the shortest wavelength to be recorded and yet is adapted to produce a shape of groove which, after conversion into a disc record, can properly be played by a stylus engaging the correspondingly shaped groove in a derivative disc.
A further proposal for recording mechanically reproducable high frequency signals on carriers is disclosed in U.S. Pat. No. 3,737,589 issued 5th June 1973 but bearing a foreign priority date of 1st Aug. 1970. The method disclosed in this Patent consists of using a photoresist which is variably softened by an electron beam or a laser beam by an amount corresponding to the intensity of the beam. The Patent discloses the movement of the beam so that it acts as a cutting surface, for which purpose the beam is transversely deflected at a frequency of approximately 15 MHz and is, in addition to being modulated in accordance with the frequency of the signal to be recorded, modulated at a frequency twice the transverse deflection frequency. Although this method as applied to a photoresist can produce a groove of approximately V-shaped cross-section the application of the method to deposition of electric charge by an intensity modulated beam of electrons is insufficient since as will be explained hereinafter it is normally appropriate to control the beam of electrons so that every point on the film which corresponds to a point within the width of the groove that is formed receives charge in order that the film be deformed properly. Hereinafter there is disclosed an embodiment of the invention in which the beam of electrons is deflected transversely at a suitable rate but it is important in such a case to ensure that the rate of transverse oscillation of the beam is sufficient to deposite charge at least twice on every point that is within the width of the spiral track so formed. Moreover, in such an embodiment it is usually necessary to provide, as explained hereinafter, blanking of the beam at the extremities of its transverse movement. The arrangement shown in U.S. Pat. No. 3,737,589 is accordingly not adapted to producing a deposited track of charge which, subject to softening of the thermoplastic film, deforms the thermoplastic film into a shape whose contours correspond to the signal to be recorded and which define a properly shaped groove.
Another proposal for recording video signals on a disc is disclosed by W. R. Johnson et al in U.S. Pat. No. 3,361,873 issued 2nd Jan. 1968. In that method a disc of sensitive material is scanned in a spiral path by a modulated beam of electrons in order to produce a spiral recording which photographically corresponds to the signal that modulates the beam of electrons. This arrangement is substantially different from the present invention and the aforementioned Patent gives no guidance on the particular parameters to be adopted for the control of the electron beam wherein it is desired to produce a spiral, modulated groove by the combined effects of deposition of charge and softening of the thermoplastic film.
Another proposal for recording signals on a disc is disclosed by Lemoine in U.S. Pat. No. 2,897,710 issued 4th Aug. 1959. In that Patent a light source produces an optical beam which is directed towards a photosensitive disc and the disc is rotated and translated relative to the source in order to produce a spiral groove. This expedient is substantially different from the deposition of electric charge and the production of a groove by the combined effect of softening the film by deposition of charge and the Patent gives no guidance on how to produce a spiral groove which is substantially wider than the shortest wavelength to be recorded.